1. Field of the Invention
The present invention relates to an optical tomograph that obtains optical tomographic images by OCT (Optical Coherence Tomography) measurement.
2. Description of the Related Art
Conventionally, ultrasonic tomographs that employ ultrasonic waves are employed to obtain tomographic images of the interiors of body cavities. Optical tomographs that employ light interference of low interference light have also been proposed (refer to Japanese Unexamined Patent Publication No. 2003-172690, for example). The optical tomograph disclosed in Japanese Unexamined Patent Publication No. 2003-172690 is that which obtains tomographic images by time domain measurement. In this optical tomograph, a probe is inserted through the forceps opening and the forceps channel of an endoscope, to guide a measuring light beam into a body cavity.
Specifically, a low coherence light beam emitted from a light source is divided into a measuring light beam and a reference light beam. Thereafter, a reflected light beam, which is the measuring light beam reflected by a measurement target when the measuring light beam is irradiated onto the measurement target, is guided to a combining means. Meanwhile, the reference light beam is guided to the combining beams after the optical path length thereof is changed. The combining means combines the reflected light beam and the reflected light beam, and the resulting interference light beam is measured by heterodyne detection or the like. Time domain measurement utilizes the fact that interference light beams are detected when the optical path length of the measuring light beam and the reflected light match the optical path length of the reference light beam. The measurement position (measurement depth) within measurement targets beam is changed, by varying the optical path length of the reference light beam.
Recently, frequency domain OCT measurement, which obtains optical tomographic images at high speeds without sweeping the optical path length of a reference light beam, by spatially or temporally spectrally analyzing an interference light beam, has been proposed (refer to U.S. Pat. Nos. 5,565,986 and 6,377,349, for example). SD-OCT (Spectral Domain OCT) measurement that spectrally decomposes an interference light beam spatially, and SS-OCT (Swept Source OCT) measurement that spectrally decomposes an interference light beam temporally, are known as methods of measurement for frequency domain OCT.
In SD-OCT measurement, the frequency of a light beam emitted from a light source is spatially spectrally decomposed, and detection of an interference light beam is performed at single moments in time. For example, an SD-OCT optical tomograph divides a wide band low coherence light beam emitted by a light source into a measuring light beam and a reference light beam by a Michelson interferometer. Then, the measuring light beam is irradiated onto a measurement target, and a reflected light beam, which is the measuring light beam reflected by the measurement target, is combined with the reference light beam, to obtain an interference light beam. Thereafter, the interference light beam is decomposed into different frequency components. The channeled spectra of the decomposed interference light beam undergo Fourier analysis, and tomographic images are obtained without scanning in the depth direction.
In SS-OCT measurement, a coherent light beam emitted from a light source is divided into a measuring light beam and a reference light beam, and a reflected light beam, which is the measuring light beam reflected by a measurement target, is combined with the reference light beam. Optical tomographic images are obtained, based on the intensity of an interference light beam formed by interference between the reflected light beam and the reference light beam. In SS-OCT measurement, an interference light beam is detected while varying the frequency of the light beam emitted from the light source over time. An SS-OCT optical tomograph sweeps the frequency of a laser beam which is emitted from a light source, using a Michelson interferometer, for example. Reflected light beams of each wavelength are caused to interfere with the reference light beam. The intensities of reflected light beams at depth positions within a measurement target are obtained from interferograms of optical frequency bands, and tomographic images are obtained employing the detected intensities.
In this manner, frequency domain OCT apparatuses are capable of obtaining reflectance, that is, tomographic data, at each depth position, by performing frequency analysis.
In frequency domain OCT apparatuses, positions relative to a reference point, at which an optical path difference is zero, can be obtained. Therefore, in principle, it is not necessary to match the combined optical path length of a measuring light beam and a reflected light beam with the optical path length of a reference light beam. In practice, however, in the case that an optical path difference is great, the spatial frequency of an interference signal tends to be amplified. Therefore, the maximum optical path length difference is necessarily defined by the spatial resolution or a temporal resolution of a photodetector that detects an interference light beam. That is, in an SD-OCT apparatus, the range of optical path length differences for which tomographic data can be obtained is determined by the intervals between photodiodes, which is the spatial resolution, in the case that a photodiode array is employed as a photodetector. Similarly, in an SS-OCT apparatus, the range of optical path length differences for which tomographic data can be obtained is determined by a sampling interval of a photodetector, which is the temporal resolution.
For this reason, it is necessary to adjust an optical path difference at least prior to obtainment of optical tomographic data, such that a measurement target is included within a range of optical path length differences, within which optical tomographic data can be obtained. Accordingly, an optical path length adjusting means is generally included within the optical path of the measurement light beam or the reference light beam in optical tomographs. Normally, an operator sets an optical path length prior to obtaining a desired optical tomographic image, obtains the optical tomographic image, then causes the optical tomographic image to be displayed on a display device. The operator views the displayed optical tomographic image, then adjusts the optical path length adjusting means manually such that a measurement target is included in the next optical tomographic image. Thereafter, the desired optical tomographic image is obtained.
However, the optical lengths of a measuring light beam and a reference light beam may change due to temperature fluctuations. In addition, if optical fibers are employed to guide the measuring light beam and the reference light beam, there are cases in which bending of the optical fibers can cause changes in the optical path lengths thereof. For these reasons, there are cases in which an optical tomographic image of a desired measurement target cannot be favorably obtained, due to shifting in the obtainment position (depth) caused by changes in the optical path lengths during obtainment of the optical tomographic image.